function realu = U3(x,t)
        s = 2*pi*pi/3*ones(size(x));
        ssign = 1;
        for k = 1:201
          s = s + 4*ssign*exp(-k^2*t)*cos(k*x)/k/k;
          ssign  = -ssign;
        end
        realu = s;
end
